Mobile satellite systems will allow mobile earth stations, namely vehicles, aircraft or boats, to communicate via satellites. Such a system, known as MSAT, is being developed for North America with the intention of providing services such as mobile telephone, mobile radio and mobile data transmission. Antennas which have been proposed for use with MSAT mobile earth stations are typically large and/or expensive. One such antenna, for example, comprises a rod about one meter long and several centimeters in diameter, while another comprises a disc of about 25 centimeters in diameter and about 5 centimeters thick, which would be mounted several centimeters above the roof of the vehicle. When an antenna is mounted upon a vehicle, especially an automobile, or an aircraft, it is subject to dynamic forces caused by wind drag, inertia and so on, which can cause performance degradation. Moreover, an antenna of large size and ungainly appearance would detract from the aesthetic appearance of the automobile and could dissuade potential users from subscribing to the system.
It is desirable, therefore, for the antenna for the mobile earth station to be relatively small and unobtrusive, especially if it is to be mounted upon an automobile.
It has also been proposed to use antennas which use electronically-phased steering, but they are complicated and expensive to build. Also, these antennas tend to be relatively large so as to compensate for losses in the phasing circuits.
The present inventor prefers to use a mechanically steered antenna with a directional active antenna element. Although they can be relatively small, known mechanically steered antennas require precision machined parts, which would tend to make them expensive and unreliable. They also would require relatively large radiator elements to compensate for losses in their rotary couplings. For a mechanically steered antenna to be viable, improvement is required in the coupling efficiency and the gain of the radiator element. The present inventor's copending application Ser. No. 08/024,461, filed concurrently herewith, now U.S. Pat. No. 5,432,524, the entire contents of which are incorporated herein by reference, addresses the problem of rotary couplings for such mechanically steered antennas.
So far as the active antenna element is concerned, a generally helical shape is beneficial because it is highly directional, broadband, has high gain and has a high axial ratio. Helical antennas are, of course, well known. In an article entitled "A New Helical Antenna Design for Better On-and Off-Boresight Axial Ratio Performance", IEEE Transactions on Antennas and Propagation, Vol. AP-28, No. 2, March 1980, Cheng Donn discloses several helical antennas. One has a partially tapered end, with sixteen normal turns and two turns on the taper, while his preferred design has sixteen normal turns and between four and eight turns on the taper. Such an antenna element would, however, be unsuitable for a compact mechanically steered antenna.
Short, cylindrical helical antennas are discussed by H. Nakano et al in several articles, namely "Radiation Characteristics of Short Helical Antenna and its Mutual Coupling", Electronics Letters, Mar. 1, 1984, Vol. 20, No. 5; "Backfire Radiation from a Monofilar Helix with a Small Ground Plane", IEEE Transactions on Antennas and Propagation, Vol. 36, No. 10, October 1988; "Extremely Low-Profile Helix Radiating a Circularly Polarized Wave", IEEE Transactions on Antennas and Propagation", Vol. 39, No. 6, June 1991. From these articles, it is apparent that a highly shortened helix of 1.5 to 2 turns with a pitch angle of the order of 4-8 degrees can be an efficient radiator. The performance figures disclosed by Nakano et al, however, are for infinite ground planes. When mounted upon small ground planes, as required in practice for mobile earth stations, highly shortened helical antennas are difficult to impedance-match, have a high return loss, and do not have sufficient gain to meet current (MSAT) mobile satellite system requirements.
Conical antennas have been disclosed in U.S. Pat. No. 3,283,332, (Nussbaum) issued November 1966; U.S. Pat. No. 4,675,690 (Hoffman) issued Jun. 23, 1987; and Canadian patent number 839,970 (R. Gouillou et al), issued Apr. 21, 1970. As disclosed especially by Hoffman and by Gouillou et al, a lightweight antenna may be made economically by forming a conductor onto a substrate by means of a photoresist-type etching process and rolling the substrate to form a cone or, as in one of Gouillou et al's examples, a trunco-conical shape. Each of these conical antennas would be unsuitable for mobile earth terminals due to one or more of the following: poor directivity; large size; poor axial ratio; insufficient gain; poor bandwidth; frequency sensitive performance.
Despite this extensive state-of-the-art in antennas, the technical requirements for MSAT are so stringent that the MSAT specifications envisage the use of two different antennas for the mobile earth stations. One would be used by mobile earth stations operating in the more northerly latitudes of the satellite's coverage area and the other in the more southerly latitudes. This duplication is undesirable. It would, of course, be preferable for a single antenna to be used for all latitudes.